1. Field of the Invention
The present invention relates to a magnetic random access memory having memory cells, each using a magnetoresistance element that stores data by means of the magnetoresistance effect.
2. Description of the Related Art
“Magnetic random access memory” (which will be referred to as MRAM) is a generic name of solid memories that can rewrite, hold, and read record information, as the need arises, by utilizing the magnetization direction of a ferromagnetic body used as an information recording carrier.
In general, each of the memory cells of the MRAM has a structure in which a plurality of ferromagnetic bodies are stacked one on the other. Information recording is performed by assigning units of binary information “1” and “0” respectively to parallel and anti-parallel states, i.e., the relative positions in magnetization, of the plurality of ferromagnetic bodies forming each memory cell. When record information is written, the magnetization direction of a ferromagnetic body of each cell is inverted by a magnetic field generated by electric currents fed through write lines, which are disposed in a criss-cross fashion. The MRAM is a nonvolatile memory, which, in principle, has zero power consumption during record holding, and record holding is maintained even after power off. Record information is read by utilizing the so-called magnetoresistance effect, in which the electric resistance of each memory cell varies in accordance with the angle between the magnetization direction of a ferromagnetic body in each memory cell and the sense current, or angle between the magnetization directions of a plurality of ferromagnetic layers.
The MRAM has many advantages in function, as shown in the following (1) to (3), as compared to conventional semiconductor memories using a dielectric body. (1) It is completely nonvolatile, and allows the number of rewriting operations to be more than 1015. (2) It allows nondestructive reading, and requires no refreshing operation, thereby shortening read cycles. (3) As compared to memory cells of the charge accumulation type, it has a high radiation-tolerance. The MRAM may have an integration degree per unit area, and write and read times, almost the same as those of the DRAM. Accordingly, it is expected that the MRAM will be applied to external recording devices for portable equipment, hybrid LSIs, and primary storage for personal computers, making the most of the specific feature of nonvolatility.
At present, feasibility studies are being carried out regarding practical use of the MRAM, in which each memory cell employs, as a magnetoresistance element, an MTJ (Magnetic Tunnel Junction) element that forms a ferromagnetic tunnel junction (for example, Roy Scheuerlein, et al., “A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell”, 2000 ISSCC Digest of Technical Papers, US, February 2000, pp. 128-129). The MTJ element is formed mainly of a three-layered film, i.e., ferromagnetic layer/insulating layer/ferromagnetic layer, in which an electric current flows by tunneling through the insulating layer. The electric resistance of the junction varies in proportion to the cosine of the relative angles in magnetization of the two ferromagnetic metal layers. The resistance becomes maximum when the magnetization directions are anti-parallel with each other. This is called the TMR (Tunneling Magneto-Resistance) effect. For example, in the case of NiFe/Co/Al2O3/Co/NiFe, a magnetoresistance change rate of more than 25% is observed with a low magnetic field of 50 Oe or less.
As a structure of the MTJ element, there is known a so-called spin valve structure type, in which an anti-ferromagnetic body is disposed adjacent to one of two ferromagnetic bodies to fix its magnetization direction, so as to improve the magnetic field sensitivity (for example, M Sato, et al., “Spin-Valve-Like Properties of Ferromagnetic Tunnel Junctions”, Jpn. J. Appl. Phys., 1997, Vol. 36, Part 2, pp. 200-201). There is also known a type in which double tunnel barriers are disposed to improve bias dependency of the magnetoresistance change rate (for example, K Inomata, et al., “Spin-dependent tunneling between a soft ferromagnetic layer and hard magnetic nano particles”, Jpn. J. Appl. Phys., 1997, Vo. 36, Part 2, pp. 1380-1383).
However, in order to develop MRAMs having an integration degree of Class-Gb, there are several problems that still need to be solved. One of them is that the writing current needs to be reduced. In the conventional MRAM, an electric current is caused to flow through an interconnection line to generate a magnetic field, thereby inverting the magnetization direction of the record layer of an MTJ element. The magnetic field intensity generated by the interconnection line varies, depending on the value of the electric current fed through the interconnection line, and the distance between the interconnection line and MTJ element. According to conventional reports, the magnetic field intensity is about several Oe/mA. The threshold for inverting the magnetization direction of the record layer of the MTJ element, which will be defined as switching magnetic field Hsw, increases in inverse proportion to the size of the MTJ element in the hard magnetization axis direction, which will be defined as cell width w.Hsw=Hsw0+A/w  (1)where the conventionally known value of A is 10 to 20 (Oe·μm).
In light of the reliability of an interconnection line, electro-migration imposes a restriction thereon. The rate of electro-migration depends on the electric current density in the interconnection line. In an Al—Cu interconnection line and a Cu interconnection line presently used in LSI manufacture, the upper limit of electric current density is about 10 mA/μm2 and 100 mA/μm2, respectively. In consideration of manufacture under a 0.1-μm rule necessary for realizing the integration degree of Class-Gb, even where an interconnection line is formed of a Cu interconnection line, the upper limit of electric current value acceptable for the interconnection line is about 1 mA, and the value of a magnetic field generated thereby is several Oe. On the other hand, the switching magnetic field for an MTJ element with a size of about 0.1 μm is several 10 Oe or more, in accordance with the formula (1). As a consequence, it is very difficult to realize an MRAM of the Class-Gb using present techniques.
In order to solve this problem, a device with a keeper layer or yoke structure (magnetic circuit), which is made of a magnetic material having a high magnetic permeability and is disposed around an interconnection line, has been proposed (for example, U.S. Pat. Nos. 5,940,319 and 5,956,267, International Publication WO 00/10172, and Jpn. Pat. Appln. KOKAI Publication No. 8-306014). These devices are designed to cause the magnetic flux generated around the interconnection line to converge into the keeper layer or yoke structure, thereby intensifying the magnetic field generated around an MTJ element, so as to reduce the writing current value.
Of the above, the “yoke structure”, as shown in FIG. 9, is a structure that can be practically manufactured under the 0.1-μm rule necessary for realizing the integration degree of Class-Gb. As shown in FIG. 9, electric current drive lines 2 are respectively and electrically connected to two MTJ elements 1. Each of the electric current drive lines 2 includes a line core portion 3 made of a low-resistivity metal, such as Al, a barrier metal film 4 made of, e.g., TaN, and a high magnetic permeability film 5 made of, e.g., Ni. The high magnetic permeability film 5 functions as a yoke for holding a magnetic field. The barrier metal film 4 prevents inter-diffusion of metals between the line core portion 3 and high magnetic permeability film 5, thereby improving the reliability of the MRAM.
As described later in more detail, the present inventor has found that, where the yoke structure shown in FIG. 9 is applied to an MRAM in practice, several problems arise, such as an increase in the interconnection line resistivity, and an increase in the connection resistivity.